1. Field of the Invention
The present invention relates to an charge exchanger, to a method by which the polarity of an ion beam is changed by the charge exchanger, and to an ion implantation system comprising the charge exchanger. More particularly, the present invention relates to an charge exchanger which produces Mg (magnesium) molecules for collision with an ion beam, which collision acts to change the polarity of the beam, and to a method of controlling the amount of the Mg colliding with the ion beam so as to regulate the rate at which the polarity of the ions of the beam is changed.
2. Description of the Related Art
An charge exchanger changes the polarity of ions from positive to negative ions, or from negative to positive. One of the main factors in assessing the efficiency of the charge exchanger is the rate at which the polarity of the ions is changed. One type of system that employs an charge exchanger is an ion implantation system used in the fabricating of semiconductor devices.
In general, ion implantation refers to technology in which ions are injected into a given target. In a conventional ion implantation technique, the energy supplied to the ions is sufficient to accelerate the ions to such a degree that they can penetrate the target surface.
With such a technique, the concentration of impurities can be maintained within a range of 1014 to 1018 atoms/cm3. Such a technique is widely used for implanting ions into a given target material because it controls the concentration of impurities better than other impurity implantation techniques.
An ion implantation system generally includes at least the following components: a vacuum apparatus, an ion source, an ion extractor, an charge exchanger, a mass analyzer, an accelerator tube, and a final process station. The system is designed to supply varying levels of high voltage to effect ion decomposition, extraction, and acceleration. During the ion implantation process, the gas molecules supplied from the ion source collide with hot electrons and are extracted by an electric field formed by the applied voltage. The extracted ions form an ion beam. The ions are selectively analyzed for their charge while the ion beam is diffracted, and the ion beam is then accelerated sufficiently to penetrate a wafer to an intended depth.
The above-described conventional ion implantation system generally has a structure as shown in FIG. 1. In the ion implantation system 100, a positive ion beam 3 is extracted from an ion source 1 through an extractor 2, and the positive ion beam 3 is converted into a negative ion beam at an charge exchanger 4. The negative ion beam is supplied to a mass analyzer 5. In the mass analyzer 5, the ion beam is diffracted by a magnetic field and is analyzed.
Next, the ion beam 3 is accelerated by an accelerator 6. The energy by which the ion beam is made to travel is doubled by the accelerator 6. The accelerated ion beam passes through a stripper 7 which changes the polarity of the ion beam from negative to positive. The accelerating ion beam then passes through a magnet 9 of a final processing (ion implantation) station 8. The magnet 9 deflects the beam onto a wafer 11 held on a disk 10.
While some of the ions are injected into the wafer, the remainder of the ion beam 3 passes through the disk 10 and collides with a Faraday Cup 12. Here, the dosage is measured by the flow of the electrons supplied from the ground.
As described above, the implantation system 100 effects two ion exchanges, one at the charge exchanger 4 where the polarity of the ion beam is changed from positive to negative, and the other at the stripper 7 where the polarity of the beam is changed from negative to positive.
One example of a conventional charge exchanger 20 is shown in FIG. 2. In this device, a solid piece of Mg 26 is installed in a casing 22 through a hole in the casing 22. A heater 28 is inserted through the center of the piece of Mg 26.
The heater 28 is heated to produce a temperature of 450xc2x0 C. or more. At this time, a vacuum is formed inside the charge exchanger 20. The heat vaporizes the Mg. The resulting gaseous magnesium molecules collide with the ion beam 3 extracted from the ion source 1. Then, the ion beam 3 receives the electrons from the magnesium molecules and consequently acquires a negative net charge. For example, BF2+ ions are converted to BF and Fxe2x88x92 ions, and BF+ ions to B and Fxe2x88x92 ions, B+ to Bxe2x88x92 ions, and F+ to Fxe2x88x92 ions.
The ion exchange rate can be measured as the percentage of the ions making up the positive ion beam which become negative. The conventional charge exchanger 20 effects a very low ion exchange rate of less than 5%.
A low ion exchange rate means that a small amount of the magnesium molecules bonded with the extracted ions. The magnesium molecules, which have not bonded with the extracted ions, flow inside the charge exchanger 20, and when the ion implantation is completed, and the inner temperature of the charge exchanger 20 cools to less than 250xc2x0 C., the molecules attach to the inside of the charge exchanger 20 and contaminate the same. The molecules especially contaminate an area around the vacuum seal (not shown). The charge exchanger must therefore be cleaned periodically to remove the contaminants.
In addition, heating the magnesium 26 to a high enough temperature to form gaseous magnesium molecules requires a large amount of power. This contributes to the high cost of running the facility.
An object of the present invention is to provide an charge exchanger capable of establishing a high exchange rate which contributes to the efficiency of the facility in which the charge exchanger is employed.
To achieve this object, the charge exchanger of the present invention comprises an ion exchange member provided in the path of an ion beam, and through which member the ion beam can pass, and a charge conversion material coating the member. The charge conversion material is made up of charged particles that will change the polarity of the ion beam when ions of the ion beam collide with them.
The charge exchanger further comprises a drive means in the form of a motor or the like for advancing the member while it is being impinged by the ion beam in order to provide new particles of charge conversion material for collision with the ions of the ion beam.
The ion exchange member is in the form of tape wound around a pair of rotary shafts. The motor is connected to one of the shafts to rotate the same and thereby advance the ion exchange member. Means may be provided for controlling the speed of the motor. In this case, the ion exchange member is provided with indicia, and a sensor senses the indicia as the ion exchange member is being advanced. The indicia can be magnetic imprints formed on the tape or a series of holes formed in the tape. Information from the sensor is used as feedback to control the speed of the motor.
Another object of the present invention is to provide a method in which the ion exchange rate can be controlled to effect a precise ion implantation, based on the dosage of ions injected into a target or on preselected ion injection parameters.
To achieve this object, the present invention provides a method in which the ion exchange rate is controlled during ion implantation by predetermining an ion dosage on the basis of the desired ion implantation process to be carried out, directing an ion beam to the wafer through the ion exchange member, and controlling the speed at which the ion exchange member is advanced to establish an ion exchange rate that produces an ion beam of a dosage that will effect the desired ion implantation process.
The speed is selected by measuring the dosage of the ion beam injected into the wafer or determining the ionization state of the ion beam required to effect an appropriate ion exchange rate, and based on this information, advancing the ion exchange member at a speed which establishes the necessary ion exchange rate.
A further object of the present invention is to provide an ion implantation system in which the ion exchange rate is high, the contamination of the charge exchanger is kept low, and which requires a relatively low amount of power to operate.
To achieve this object, the present invention provides an ion implantation system comprising an extractor for forming an ion beam from a source of ions, a mass analyzer for mass-analyzing the ion beam, an charge exchanger for changing the polarity of the ion beam, an accelerator for supplying the ion beam with energy in order to accelerate the ion beam, and an ion implantation station at which an ion beam is injected into a wafer, the charge exchanger comprising a casing having an inlet and an outlet through which the ion beam passes, an ion exchange member disposed in the path of the ion beam and having charge conversion material which will change the polarity of the ion beam when the ion beam collides therewith, and drive means for advancing the ion exchange member to provide a new surface of charge conversion material against which the ion beam will collide.